Remediation of PCB-contaminated soil using a combination of mechanochemical method and thermal desorption.

The combination of mechanochemical method and thermal desorption for remediating polychlorinated biphenyls (PCBs) in contaminated soil was tested in this study. The effects of grinding time and heating time on PCB removal efficiency were investigated. The contaminated soil, mixed with CaO powder at a weight ratio of 1:1, was first ground using a planetary ball mill. After 4 h of grinding, the total PCB concentration and its toxic equivalence quantity (TEQ) decreased by 74.6 and 75.8%, respectively. Then, after being heated at 500 °C for 60 min, the residual PCBs in mechanochemical + thermal treated soil decreased to 247 ng/g, resulting in a removal efficiency of 99.95%. The removal effect can be promoted by longer grinding time and heating time; however, increased energy consumption was inevitable. The combination of grinding time and heating time should be optimized in a practical remediation process.

Adsorption of polychlorinated dibenzo-p-dioxins/dibenzofurans on activated carbon from hexane.

Activated carbon is widely used to abate dioxins and dioxin-like compounds from flue gas. Comparing commercial samples regarding their potential to adsorb dioxins may proceed by using test columns, yet it takes many measurements to characterise the retention and breakthrough of dioxins. In this study, commercial activated carbon samples are evaluated during tests to remove trace amounts of dioxins dissolved in n-hexane. The solution was prepared from fly ash collected from a municipal solid waste incinerator. The key variables selected were the concentration of dioxins in n-hexane and the dosage of activated carbon. Both polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) showed very high removal efficiencies (94.7%-98.0% for PCDDs and 99.7%-99.8% for PCDFs). The presence of a large excess of n-hexane solvent had little effect on the removal efficiency of PCDD/Fs. The adsorbed PCDD/Fs showed a linear correlation (R(2) > 0.98) with the initial concentrations. Comparative analysis of adsorption isotherms showed that a linear Henry isotherm fitted better the experimental data (R(2) = 0.99 both for PCDDs and PCDFs) than the more usual Freundlich isotherm (R(2) = 0.88 for PCDDs and 0.77 for PCDFs). Finally, the results of fingerprint analysis indicated that dioxin fingerprint (weight proportion of different congeners) on activated carbon after adsorption did not change from that in hexane.

Removal efficiencies for 136 tetra- through octa-chlorinated dibenzo-p-dioxins and dibenzofuran congeners with activated carbons.

In this study, the removal efficiency of 136 tetra- to octa-chlorinated dibenzo-p-dioxin (CDD)/furan (F) congeners from a nitrogen + oxygen carrier gas was studied using a laboratory-scale, fixed bed adsorption system. Two kinds of activated carbon with dissimilar pore structures were used as adsorbents. The total concentration of polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs) in the source gas was 541 ng/Nm(3) and that of the 17 toxic 2,3,7,8-substituted PCDD/Fs 96.35 ng/Nm(3), accounting for 17.8% of the total original weight amount. Their toxic equivalent quantity (TEQ) was 8.31 ng I-TEQ/Nm(3). For both activated carbons, the removal efficiencies of the ten PCDD/F homologue groups rise with chlorine substitution number. The removal efficiencies vary approximately as a power function of vapor pressure (correlation coefficients r(2) = 0.93 and 0.81, respectively). Competitive adsorption and desorption occur as adsorption time went on, causing elution of the lower chlorinated homologues, i.e. tetra-CDD/F and Penta-CDD/F congeners. In addition, there are significantly different concentration distributions for isomers in the same homologue groups. However, their removal efficiencies have weak correlation with their initial concentrations. The correlation coefficients are from -0.47 to 0.32 and from -0.57 to 0.46 respectively for the two kinds of activated carbons.

Ozone-enhanced oxidation of PCDD/Fs over V(2)O(5)-TiO(2)-based catalyst.

The catalytic oxidation of PCDD/Fs (polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans) vapors was studied in a temperature range of 180-220°C on a honeycomb V2O5-TiO2-based catalyst, in the presence and the absence of ozone. A stable dioxin-generating system was established to support the experimental program and this system could adjust the concentration of PCDD/Fs by injecting appropriate mother liquors. At 220°C the removal efficiency(1) (RE) of PCDD/Fs reaches up to 97% and the degradation efficiency (DE) up to 90%. Both values diminish at lower operating temperatures. In the presence of ozone, however, these values rise to 99% and 98% at 220°C. Especially at low temperatures the effect of ozone is obvious. Catalytic oxidation with ozone thus offers a low-temperature solution to achieve higher rates and low activation energies. The morphology and microstructure of the catalysts changes after ozone treatment and some of their characteristics seem closely related with DE-values.

Dioxins and their fingerprint in size-classified fly ash fractions from municipal solid waste incinerators in China--mechanical grate and fluidized bed units.

The distribution of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs), in brief dioxins, has seldom been addressed systematically in fly ash from municipal solid waste incinerators (MSWIs). This study shows the amount and fingerprint of PCDD/Fs in fly ash from four different Chinese MSWIs, that is, three mechanical grate units and one circulating fluidized bed unit. In these fly ash samples, dioxins-related parameters (international toxic equivalent quantity, total amount of PCDD/Fs, individual isomer classes, and 17 toxic 2,3,7,8-substituted congeners) all tend to increase with decreasing particle size for mechanical grate incinerators, yet only for the finest fraction for fluidized bed units. Moreover, the fluidized bed incinerator seems superior to grate incineration in controlling dioxins, yet a comparison is hampered by internal differences in the sample, for example, the fluidized bed fly ash has much lower carbon and chlorine contents. In addition, the presence of sulfur from mixing coal as supplemental fuel to the MSW may poison the catalytic steps in dioxins formation and thus suppress the formation of dioxins. With more residual carbon and chlorine in the fly ash, it is easier to form dioxins during cooling. Nevertheless, there is no apparent relation between Fe, Cu, and Zn contents and that of dioxins in fly ash.

Gas/particle partitioning of atmospheric PCDD/Fs in a satellite town in Eastern China.

Gas/particle partitioning of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in ambient air was investigated in a satellite town in Eastern China from April 2007 to January 2008 comprehending large temperature variations (from 3 to 34 degrees C, daily average). Molecular weight, molecular structure and ambient temperatures are the three major factors that govern the gas/particle partitioning of atmospheric PCDD/Fs throughout the year. Generally, good agreements were obtained (except for winter) between measured particulate fractions and theoretical estimates of both the Junge-Pankow adsorption model and Harner Bidleman absorption model using different sets of subcooled liquid vapor pressure (P(L)(o)) and octanol-air partition coefficient (K(oa)), respectively. Models utilizing P(L)(o) estimates, derived from gas chromatographic retention indices (GC-RIs), are more accurate than that of entropy-based. Moreover, during winter, the K(oa)-based model using the GC-RIs approach performs better on lower chlorinated PCDD/Fs than that of P(L)(o)-based. Furthermore, possible sources of mismatch between measured and predicted values in winter (3-7 degrees C) were discussed. Gas adsorption artifact was demonstrated to be of minor importance for the phenomena observed. On the other hand, large deviations of slopes (m(r)) and intercepts (b(r)) in logK(p) vs. logP(L)(o)(Pa)/logK(oa) plots from theoretical values are observed in the literature data and these are found to be linearly correlated with ambient temperatures (P<0.001) in this study. This indicates that the non-equilibrium partitioning of PCDD/Fs in winter may be significantly influenced by the colder temperatures that may have slowed down the exchange between gaseous and particulate fractions.

Concentrations, profiles, and sources of atmospheric PCDD/Fs near a municipal solid waste incinerator in Eastern China.

Ambient air monitoring of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) was performed to investigate their concentrations, profiles, and sources near a municipal solid waste incinerator (MSWI) in Eastern China. The PCDD/F concentrations varied over a factor of 51.4 from 0.059 to 3.03 pg I-TEQ m(-3) (or over 41.4 from 3.96 to 164 pg m(-3)) and fell at the higher end of the range in recorded peer investigations around the world. Almost all ambient air samples show a unique TCDF-dominanthomologue pattern, characterized by decreasing concentrations with increasing levels of chlorination (except for OCDD). By contrast, the dominant congener in terms of concentration profiles shows temporal and spatial variations among the OCDD, OCDF, and 1,2,3,4,6,7,8-HpCDF. The results from a congener-specific factor analysis between ambient air and PCDD/F emission sources, as well as U.S. EPA Chemical Mass Balance (CMB8.2) and Industrial Source Complex Short Term Version 3 (ISCST3) modeling, suggest that the deterioration of air quality in the study area is primarily attributed to open burning of wastes (OB), followed by hot water boilers (HWBs), traffic, and the MSWI.

Agricultural soil monitoring of PCDD/Fs in the vicinity of a municipal solid waste incinerator in Eastern China: temporal variations and possible sources.

The temporal variations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in 33 agricultural soil samples in the vicinity of a municipal solid waste incinerator (MSWI) in Eastern China were determined one year after the initial investigation in 2006. The soil PCDD/F concentrations in 2007 ranged from 73.6 to 377 ng kg(-1) (0.60-6.38 ng I-TEQ kg(-1)). During 2006-2007, the overall soil PCDD/F levels increased significantly, i.e., 33% and 39% for total concentration and I-TEQ (median value), respectively. Moreover, soils in the study area are proved to be almost free from previously suspected PCDD/F sources, i.e., pentachlorophenol/sodium penta-chlorophenate (PCP/PCP-Na) and 1,3,5-trichloro-2-(4-nitrophenoxy) benzene (CNP) contaminations. Furthermore, the results from a congener-specific factor analysis between soils (collected in two investigations) and dioxin emission sources suggest that diffuse sources including open burning of wastes, traffic and hot water boilers are major contributors that are responsible for the accumulation of PCDD/Fs in soils. By contrast, the impact of the presumably major PCDD/F source identified in our previous study, i.e., the MSWI, seems to be limited.

Gasification characteristics of MSW and an ANN prediction model.

Gasification characteristics make up the important parts of municipal solid waste (MSW) gasification and melting technology. These characteristics are closely related to the composition of MSW, which alters with climates and seasons. It is important to find a practical way to predict gasification characteristics. In this paper, five typical kinds of organic components (wood, paper, kitchen garbage, plastic, and textile) and three representative types of simulated MSW are gasified in a fluidized-bed at 400-800 degrees C with the equivalence ratio (ER) in the range of 0.2-0.6. The lower heating value (LHV) of gas, gasification products, and gas yield are reported. The results indicate that gasification characteristics are different from sample to sample. Based on the experimental data, an artificial neural networks (ANN) model is developed to predict gasification characteristics. The training and validating relative errors are within +/-15% and +/-20%, respectively, and predicting relative errors of an industrial sample are below +/-25%. This indicates that it is acceptable to predict gasification characteristics via ANN model.

Pyrolysis characteristics of organic components of municipal solid waste at high heating rates.

The pyrolysis characteristics of six representative organic components of municipal solid waste (MSW) and their mixtures were studied in a specially designed thermogravimetric analysis apparatus with a maximum recorded heating rate of 864.8 degrees Cmin(-1). The pyrolysis behavior of individual components was described by the Avrami-Erofeev equation. The influence of final temperature on individual components was studied, and it was concluded that final temperature was a factor in reaction speed and intensity, but that it played only a limited role in determining the reaction mechanism. The interactions between different components were evaluated, and it was concluded that the interaction between homogeneous materials was minimal, whereas the interaction between polyethylene and biomass was significant.

Source identification of PCDD/Fs in agricultural soils near to a Chinese MSWI plant through isomer-specific data analysis.

Isomer-specific data were investigated in order to identify the sources of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in agricultural soils, including Fluvo-aquic and paddy soils, in the vicinity of a Chinese municipal solid waste incineration (MSWI) plant. Homologue and isomer profiles of PCDD/Fs in soils were compared with those of potential sources, including combustion sources, i.e., MSWI flue gas and fly ash; and the impurities in agrochemicals, such as the pentachlorophenol (PCP), sodium pentachlorophenate (PCP-Na) and 1,3,5-trichloro-2-(4-nitrophenoxy) benzene (CNP). The results showed that the PCDD/F isomer profiles of combustion sources and agricultural soils were very similar, especially for PCDFs, although their homologue profiles varied, indicating that all the isomers within each homologue behave identically in the air and soil. Moreover, factor analysis of the isomer compositions among 33 soil samples revealed that the contamination of PCDD/Fs in agricultural soils near the MSWI plant were primarily influenced by the combustion sources, followed by the PCP/PCP-Na and CNP sources. This implication is consistent with our previous findings based on chemometric analysis of homologue profiles of soil and flue gas samples, and identifies PCP/PCP-Na as an additional important source of PCDD/Fs in the local area. This makes the similarities and differences of isomer profiles between Fluvo-aquic and paddy soils more explainable. It is, therefore, advisable to use isomer-specific data for PCDD/F source identifications where possible.

Effects of inorganic chlorine source on dioxin formation using fly ash from a fluidized bed incinerator.

Chlorine source is indispensable for polychlorinated dibenzo-p-dioxin and furan (PCDD/F) formation during municipal solid waste (MSW) incineration. Inorganic chlorine compounds were employed in this study to investigate their effects on PCDD/F formation through heterogeneous synthesis on fly ash surfaces. A fly ash sample obtained from a fluidized bed incinerator was sieved to different size fractions which served as the PCDD/F formation sources. The capability of different metal chlorides which facilitate the formation of PCDDs/Fs was found to follow the trends: Na < Mg < K < Al < Ca, when two particle fractions of >177 miocrom and 104-125 microm were used in the experiments. However, the capability of NaCl, MgCl2 and KCl did not seem much different from each other, whereas CaCl2 and AlCl3 were much more active in PCDD/F formation. NaCl and MgCl2 were relatively effective to produce more PCDDs, while KCl, AlCl3 and CaCl2 generated more PCDFs during heterogeneous reactions occurring on fly ash. 2,3,7,8-TCDF was the most significant contributor to the toxicity of the PCDDs/Fs formed from inorganic chlorine sources. Decreasing the sizes of fly ash particles led to more active formation of PCDDs/Fs when NaCl was used as inorganic chlorine in the experiment. The highest PCDDs/Fs produced from particles with size <37 microm, while the lowest PCDDs/Fs produced from particles with size >177 microm. The toxicity generally increased with decreasing size of the fly ash particles. The formation of PCDDs was mainly facilitated by the two size fractions, 104-125 microm and <37 microm, while the formation of PCDFs was favored by the two other size fractions, >177 microm and 53-104 microm.

Laboratory-scale study of the suppression of PCDD/F emission during coal and MSW co-incineration.

The experimental test of co-incinerating Chinese raw municipal solid waste (MSW) and coal in a laboratory-scale tubular reactor was first reported in present study, and the emission of normal gas components and the effects of the S/C1 molar ratio or coal mixing percentages on polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/Fs) emission were investigated and discussed. The results indicated that OCDD was the only PCDD homologues since others like TCDD-HpCDD was hardly detected, while as the categories of PCDF homologues were comparatively much more general. The amount of PCDD was much larger than that of PCDF in all operating conditions. Since sigma PCDF/sigma PCDD<<1, the dominant role of the precursor formation was proven in our experimental conductions. With increasing the coal addition to MSW (from 0 to 16%), PCDD and PCDF were reduced considerably. Coal and MSW may suppress the PCDD/F emissions efficiently (over 95%) during the MSW incineration process. The PCDD/F suppression results of the present study could be helpful guidance to the industrial application of Chinese MSW and auxiliary coal co-incineration processes. The PCDD/F stack emission data of two industrial incinerators using co-incineration technology in China seem to have a great positive reduction of PCDDs/Fs.

Experimental study on MSW gasification and melting technology.

In order to develop municipal solid waste (MSW) gasification and melting technology, two preliminary experiments and a principle integrated experiment were fulfilled respectively. The gasification characteristics of MSW were studied at 500-750 degrees C when equivalence ratio (ER) was 0.2-0.5 using a fluidized-bed gasifier. When temperature was 550-700 degrees C and ER was 0.2-0.4, low heat value (LHV) of syngas reaches 4000-12000 kJ/Nm3. The melting characteristics of fly ash were investigated at 1100-1460 degrees C using a fixed-bed furnace. It was proved that over 99.9% of dioxins could be decomposed and most heavy-metals could be solidified when temperature was 1100-1300 degrees C. The principle integrated experiment was carried out in a fluidized-bed gasification and swirl-melting system. MSW was gasified efficiently at 550-650 degrees C, swirl-melting furnace maintains at 1200-1300 degrees C stably and over 95% of fly ash could be caught by the swirl-melting furnace. The results provided much practical experience and basic data to develop MSW gasification and melting technology.

Destruction of PCDD/Fs by gliding arc discharges.

PCDD/Fs have been become a serious issue because of their toxicological effects and associated adverse health implications. In this study, the gliding arc plasma was tested for treatment of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), which was synthesized from pentachlorophenol in atmospheric condition at 350 degrees C with or without the catalysis of CuCl2. From the experiment, we found that the destruction efficiency of PCDD/F homologues after gliding was discharge ranged from 25% to 79%. This result demonstrates that gliding arc plasma is an effective technology to decompose PCDDs/Fs in flue gas. A plausible degradation mechanism for PCDD/Fs by gliding arc was discussed. Finally, a multistage reactor structure of gliding arc was proposed to upgrade removal efficiency for PCDD/Fs.

[Research on low emission MSW gasification and melting system].

In order to eliminate the secondary pollution caused by MSW incineration, fluidized bed gasification and swirl-flow melting process is proposed which produces less emission. MSW of China can be effectively gasified in fluidized bed at about 600 degrees C. Combustion fly ash of MSW can be melted into vitrification at about 1300 degrees C and that of MSW combusted with coal can be done at about 1 400 degrees C, during which dioxins (PCDD/Fs) is decomposed over 99.99% and heavy-metals are mostly solidified. Two novel and improved MSW gasification and melting schemes are brought forward preferably: (1) The gasification and melting system based on integrated disposal of MSW. (2) The gasification and melting system of original MSW + assistant fuel. It is testified that the two schemes are suitable for China by thermo-performance analysis.

HCl emission characteristics and BP neural networks prediction in MSW/coal co-fired fluidized beds.

The HCl emission characteristics of typical municipal solid waste (MSW) components and their mixtures have been investigated in a phi 150 mm fluidized bed. Some influencing factors of HCl emission in MSW fluidized bed incinerator was found in this study. The HCl emission is increasing with the growth of bed temperature, while it is decreasing with the increment of oxygen concentration at furnace exit. When the weight percentage of auxiliary coal is increased, the conversion rate of Cl to HCl is increasing. The HCl emission is decreased, if the sorbent (CaO) is added during the incineration process. Based on these experimental results, a 14 x 6 x 1 three-layer BP neural networks prediction model of HCl emission in MSW/coal co-fired fluidized bed incinerator was built. The numbers of input nodes and hidden nodes were fixed on by canonical correlation analysis technique and dynamic construction method respectively. The prediction results of this model gave good agreement with the experimental results, which indicates that the model has relatively high accuracy and good generalization ability. It was found that BP neural network is an effectual method used to predict the HCl emission of MSW/coal co-fired fluidized bed incinerator.

[Effect of flue gas conditions on NO oxidation process by DC corona radical shower].

Using an air-H2O DC corona radical shower system, the influences of reside time of flue gas in the reactor, velocity of flue gas and NO concentration on NO oxidation process were studied. The results show that the increasing velocity of flue gas can restrain corona development and the increasing NO concentration can make discharge more easy. The reside time of flue gas in the reactor has less effect on the NO oxidation. The NO oxidation rate increased only from 54.5% to 57.6% at 2 W input power when the reside time of flue gas in the reactor increased from 8.5 s to 34.2 s. However, the velocity of flue gas has important effect on the NO oxidation. At 1.7 W x h/m3 energy density, when the velocity of flue gas increased from 1.4 cm/s to 6.3 cm/s, the NO oxidation rate dropped from 60.0% to 38.6% and the energy yield also falled from 20.8 g/(kW x h) to 13.3 g/(kW x h). Under the certain flux of humid air, NO initial concentration has a best value, which was about 100 x 10(-6) in this experiment.

Performance of PAHs emission from bituminous coal combustion.

Carcinogenic and mutagenic polycyclic aromatic hydrocarbons (PAHs) generated in coal combustion have caused great environmental health concern. Seventeen PAHs (16 high priority PAHs recommended by USEPA plus Benzo[e]pyrene) present in five raw bituminous coals and released during bituminous coal combustion were studied. The effects of combustion temperature, gas atmosphere, and chlorine content of raw coal on PAHs formation were investigated. Two additives (copper and cupric oxide) were added when the coal was burned. The results indicated that significant quantities of PAHs were produced from incomplete combustion of coal pyrolysis products at high temperature, and that temperature is an important causative factor of PAHs formation. PAHs concentrations decrease with the increase of chlorine content in oxygen or in nitrogen atmosphere. Copper and cupric oxide additives can promote PAHs formation (especially the multi-ring PAHs) during coal combustion.

[Leaching characteristics of heavy metals in MSW fly ash under different condition].

The leaching characteristic of heavy metals in MSW fly ash was studied under acidic and alkaline condition. The result showed the leaching characteristic of Pb, Zn, Ni, Cr and Hg had similar law, which was these heavy metals could leach out under acidic condition and alkaline condition (pH > 12), and the leaching amount increased with the leachate density or L/S increasing. For example, under acidic and alkaline condition, the leaching amount of Pb increased from 0.457 mg/kg, 51.142 mg/kg to 104.576 mg/kg, 59.692 mg/kg respectively when L/S increased from 20 to 40. Cd and Cu could leaching out only under acidic condition. Under acidic condition the leaching rate of Cd, Pb and Zn were higher then the others.